System for controlling the pressure of oil in a system for a continuously variable transmission

ABSTRACT

A control system for a continuously variable transmission has a line pressure control valve having ports and a spool for controlling the line pressure of oil supplied to a cylinder of a drive pulley. A hydraulic circuit is provided for supplying oil to a port of the line pressure control valve so as to shift the spool. A pressure reducing valve is provided in the hydraulic circuit for keeping the pressure in the circuit constant and a control valve is provided in the hydraulic circuit so as to control the quantity of the constant pressure oil supplied to the end of the spool.

BACKGROUND OF THE INVENTION

The present invention relates to a control system for a continuouslyvariable belt-drive automatic transmission for a motor vehicle, and moreparticularly to a system for controlling pressure applied to controlvalves in a hydraulic circuit for the transmission.

A known control system for a continuously variable belt-drivetransmission disclosed in U.S. Pat. No. 4,369,675 comprises an endlessbelt running over a drive pulley and a driven pulley. Each pulleycomprises a movable conical disc which is axially moved by a fluidoperated servo device so as to vary the running diameter of the belt onthe pulleys in dependency on driving conditions. The system is providedwith a hydraulic circuit including a pump, a line pressure control valveand a transmission ratio control valve. Each valve comprises a spool tocontrol the oil supplied to the servo devices.

The transmission ratio control valve operates to determine thetransmission ratio in accordance with the opening degree of the throttlevalve of the engine and the speed of the engine. The line pressurecontrol valve is adapted to control the line pressure in accordance withthe transmission ratio and the engine speed. The line pressure iscontrolled to prevent the belt from slipping on the pulleys in order totransmit the output of the engine.

At idling of the engine, the transmission ratio is set at a maximumvalue, and the line pressure is at a maximum value. When the acceleratorpedal of the vehicle is depressed, a clutch is engaged to start thevehicle. When the vehicle speed and engine speed reach set values undera driving condition, the transmission ratio starts to change (upshift).At that time if the engine speed is kept constant, the transmissionratio is automatically and continuously reduced at a speed which isdetermined by the line pressure, the pressure of oil supplied to theservo device of the drive pulley, and the actual transmission ratio.

In such a system, it is preferable to control the positions of thespools of the line control valve and transmission ratio control valve bycontrol oil which is supplied to an end of each spool through a controlvalve. The position of the spool is controlled by controlling thequantity of oil draining from the control valve. A pressure controlsystem is proposed, in which the control oil is obtained by raising thepressure of the drain oil discharged from the line pressure controlvalve. On the other hand, the line pressure is kept at a high value atlow engine speed as described above. However, at low engine speed, sincethe discharge of the pump is small, problems such as large reduction ofthe control pressure or extremely high control pressure may arise at lowor high engine speed. Such fluctuation of the control pressure causesmalfunction of the control valve.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a system which mayproduce control oil having a constant pressure for controlling controlvalves.

The other objects and features of this invention will be understood fromthe following description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of a continuously variable belt-drivetransmission;

FIGS. 2a to 2c are schematic diagrams showing a control system accordingto the present invention; and

FIGS. 3a to 3c are graphs showing relationships between controlpressure, line pressure and duty ratio.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the continuously variable belt-drive automatictransmission for a motor vehicle to which the present invention isapplied comprises an electromagnetic powder clutch 2 for transmittingthe power of an engine 1 to a transmission 4 through a selectormechanism 3.

The belt-drive transmission has a main shaft 5 and an output shaft 6provided in parallel with the main shaft 5. A drive pulley (primarypulley) 7 and a driven pulley (secondary pulley) 8 are mounted on theshafts 5 and 6 respectively. A fixed conical disc 7b of the drive pulley7 is integral with the main shaft 5 and an axially movable conical disc7a is axially slidably mounted on the main shaft 5. The movable conicaldisc 7a also slides in a cylinder 9a formed on the main shaft 5 toprovide a servo device. A chamber 9 of the servo device communicateswith a hydraulic circuit 20.

A fixed conical disc 8b of the driven pulley 8 is formed on the outputshaft 6 opposite the movable conical disc 8a has a cylindrical portionwhich is slidably engaged in a cylinder 6a of the output shaft 6 to forma servo device. A chamber 10 of the servo device is also communicatedwith the hydraulic control circuit 20. A drive belt 11 engages with thedrive pulley 7 and the driven pulley 8.

Secured to the output shaft 6 is a drive gear 12 which engages with anintermediate reduction gear 13 on an intermediate shaft 14. Anintermediate gear 15 on the shaft 14 engages with a final reduction gear16. The rotation of the final reduction gear 16 is transmitted to axles18 of vehicle driving wheels 19 through a differential 17.

Referring to FIGS. 2a to 2c, the hydraulic control circuit 20 includes aline pressure control valve 40 and a transmission ratio control valve50. The line pressure control valve 40 comprises a valve body 41, spool42, and ports 41a, 41b, 41c, 41d, 41e and chambers 41f and 41g. Thechamber 9 of the drive pulley 7 is applied with pressurized oil by anoil pump 21 from an oil reservoir 26 passing through a line pressureconduit 22, the parts 41a and 41e of the line pressure control valve 40,the transmission ratio control valve 50, and conduit 23. The chamber 10of the driven pulley 8 is applied with pressurized oil through a passage22b without passing through the valves 40 and 50. The movable conicaldisc 7a of the drive pulley 7 is so designed that the pressure receivingarea thereof is larger than that of the movable conical disc 8a of thedriven pulley 8. The spool 42 is applied with the pressurized oilsupplied to a chamber communicating with port the 41c through a conduit46. The spool 42 is urged to the left by the force of a spring 43provided between one end of the spool and a retainer 45 the position ofwhich is adjustable by a screw 44. The port 41a is communicated with theport 41b of a drain passage 27 in accordance with the position of a landof the spool 42.

The transmission ratio control valve 50 comprises a valve body 51, spool52, and spring 55 for urging the spool in the downshift direction. Aport 51b of the valve body 51 is selectively communicated with apressure oil supply port 51a or a drain port 51c in accordance with theposition of lands of spool 52. Port 51b communicates with the chamber 9through the conduit 23, and the port 51a communicates with the linepressure control valve 40 through conduit 22a. The drain port 51c iscommunicated with the oil reservoir 26 through a conduit 24 and checkvalve 25. The drain port 41b of the line pressure control valvecommunicates with the oil reservoir 26 through passage 27 and alubricating oil pressure providing valve 28 in the form of a checkvalve. The passage 27 is further communicated with the conduit 23through a check valve 30. A part of the oil in the conduit 27 issupplied to the pulley 8 from a nozzle 29 to lubricate the pulleydevice.

The system of the present invention is provided with a pressure reducingvalve 50, and solenoid operated on-off control valves 65 and 67. Thepressure reducing valve 60 comprises a valve body 61, spool 62, inletport 61a, basic pressure port 61b, drain port 61c, end chamber port 61d,spring 63 urging the spool 62 toward a chamber 61e. The basic pressureport 61b is positioned between the inlet port 61a and drain port 61c,and the spring 63 is positioned opposite the end chamber 61e against theregulator pressure. The load of the spring 63 can be adjusted by anadjust screw 64. The port 61a is connected to the pump 21 through aconduit 31 having an orifice 32 and conduit 22, and ports 61b and 61dare connected to a conduit 33. When the pressure in the conduit 33reduces, the spool 62 is shifted to the right by spring 63, so that theport 61a communicates with the port 61b to increase the pressure in theconduit 33. Thus, a constant basic pressure of oil which is lower thanthe line pressure is provided in the conduit 33.

The conduit 33 is communicated with the port 41d of the line pressurecontrol valve 40 through an orifice 34 and a passage 47. The conduit 33is also communicated with the reservoir 26 through the solenoid operatedon-off valve 65 and with an accumulator 66. Further, the conduit 33 iscommunicated with an end chamber 51f of the port 51d of the transmissionratio control valve 50 through a passage 53 and with another end chamber51g of the port 51e through a passage 54 having an orifice 35. Thesolenoid operated on-off valve 67 is connected to the passage 54downstream of the orifice 35 so as to drain the oil in the chamber ofthe drive pulley to the reservoir 26. The solenoid operated valve 65 isadapted to be operated by pulses. When energized, a valve 65a opens adrain port 65b. The solenoid operated valve 67 is the same as valve 65in construction and operation. The control valves 65 and 67 are operatedby pulses from a control unit 70. Thus, control pressures applied to theports 41d, 51e are changed by changing the duty ratios of the pulsessupplied to the control valves 65 and 67.

In the line pressure control valve 40, the relationship among springload F_(S) and line pressure PL, line pressure receiving area SL of thespool, first control pressure P_(d) F at the port 41d, and controlpressure receiving area S_(d) is as follows:

    F.sub.S +P.sub.d F·S.sub.d =PL·SL

    PL=(P.sub.d F·S.sub.d +F.sub.s)/SL

Accordingly, the line pressure PL is proportional to the controlpressure P_(d) F, as shown in FIG. 3a.

FIG. 3b shows the relationship between the duty ratio and the firstcontrol pressure P_(d) F, and FIG. 3c shows the relationship between theduty ratio and the line pressure PL.

In the transmission ratio control valve, the pressure receiving area ofthe spool 52 at chamber 51g is set to a value larger than the area atthe chamber 51f. On the other hand, a second control pressure P_(d) S inthe chamber 51g can be changed between a maximum value, which is thesame as a third control pressure P_(d) T in the chamber 51f when theduty ratio of pulses supplied to control valve 67 is 0%, and a minimumvalue by controlling the duty ratio of the pulses for operating thecontrol valve 67. The transmission ratio control valve 50 is so arrangedthat the spool 52 is at a neutral position at a middle duty ratio (forexample 50%) and is located in an oil supply position by increasing theduty ratio from the middle duty ratio because of reduction of controlpressure in the chamber 51g. Further, the speed of the movement of thespool increases with decreasing duty ratio. The spool 52 is shifted toan oil drain position by decreasing the duty ratio. It will beunderstood that when the oil is supplied to the chamber 9, thetransmission is upshifted.

The relationship between the dutry ratio (D) of the pulses applied tothe solenoid operated control valve 67 and the transmission ratio (i) isexplained hereinafter.

The transmission ratio changing speed (di/dt) is a function of flow rateQ of oil supplied to the chamber 9, and the flow rate Q is a function ofthe duty ratio D, the line pressure PL and primary pressure P_(p) in thechamber 9, namely:

    di/dt=F(Q)=F(D, PL, P.sub.p)

The line pressure PL is determined by the transmission ratio i andengine torque, and the primary pressure P_(p) is the chamber 9 isdetermined by the transmission ratio i and the line pressure PL.Accordingly, assuming the engine torque to be constant,

    di/dt=f(D,i)

    D=f(di/dt,i)

Accordingly, the duty ratio is determined by the transmission ratiochanging speed di/dt and the transmission ratio i. On the other hand,the transmission ratio changing speed di/dt is dependent on thedifference between the actual transmission ratio i and a desiredtransmission ratio id,

    di/dt=K(id-i)

where K is a coefficient.

Accordingly, if the transmission ratio changing speed di/dt isdetermined, the duty ratio D can be obtained from the speed. When theactual transmission ratio i is larger than the desired transmissionratio id (i>id), the value of di/dt is negative. In such a state, theduty ratio D is increased to reduce the pressure in the chamber 51g soas to upshift the transmission. The downshift is performed in thereverse manner.

The system is arranged to control the transmission ratio in accordancewith the above described principle. In the system, a drive pulley speedsensor 71, driven pulley speed sensor 72, throttle valve position sensor73 and engine speed sensor 74 are provided. Signals of these sensors arefed to the control unit 70. The control unit 70 produces output pulses,the duty ratios of which are determined by the input signals. The pulsesare supplied to the solenoids of the valves 65 and 67 so as to controlthe line pressure and transmission ratio.

In operation, while the vehicle is at a stop, the duty ratios of thepulses supplied to the valves 65 and 67 are small. Since the duty ratiois small, the pressure at the port 51e of the valve 50 is high.Accordingly, the spool 52 of the valve 50 is at the right end positionand the drain port 51c communicates with the chamber 9 through theconduit 23, so the chamber 10 of the driven pulley is supplied with linepressure through the conduit 22b, and the chamber 9 of the drive pulleyis drained. Thus, in the pulley and belt device of the continuouslyvariable belt-drive transmission, the driving belt 11 engages with thedriven pulley 8 at a maximum running diameter to provide the largesttransmission ratio (low speed stage). When the accelerator pedal isdepressed, the electromagnetic clutch 2 is gradually engaged,transmitting the engine power to the drive pulley 7. The power of theengine is transmitted to the output shaft 6 at the largest transmissionratio by the driving belt 11 and driven pulley 8, and furthertransmitted to the axles of the driving wheels 19. Thus, the vehicle isstarted.

At that time the line pressure PL is at the highest value by thepressure control valve 40, since the duty ratio D for the valve 65 issmall and the spool 42 of the control valve 40 is at the left endposition. When the throttle valve is opened for acceleration, the dutyratio for the control valve 67 increases so that the pressure actingforce in the chamber 51f of the control valve 50 becomes higher thanthat in the chamber 51g. Thus, the spool 52 is shifted to the left tocommunicate the port 51a with port 51b, so that oil is supplied to thechamber 9 through the conduit 23. On the other hand, the duty ratio forthe control valve 65 is increased, so that the control pressure at theport 41d of the valve 40 becomes low. The spool 42 of the valve 40 isapplied with the line pressure at the port 41c and the control pressureat the port 41d and the force of spring 43. Since the control pressurereduces, the spool 42 of the valve 40 is shifted to the right.Accordingly, the port 41a communicates with the port 41b of the drainpassage 27. Thus, line pressure reduces, and the transmission isupshifted, since oil is still supplied to the chamber 9 through thecontrol valve 50. When the opening degree of the throttle valve isreduced for deceleration, the duty ratio is reduced, thereby shiftingthe spool 52 to the right to drain the chamber 9. Thus, the transmissionis downshifted.

In accordance with the present invention, the basic pressure for controlvalves 65 and 67 is obtained from the basic line pressure at a valuelower than the line pressure. Accordingly, a constant basic pressure isprovided so that reliable control of the transmission is performed.

While the presently preferred embodiment of the present invention hasbeen shown and described, it is to be understood that this disclosure isfor the purpose of illustration and that various changes andmodifications may be made without departing from the scope of theinvention as set forth in the appended claims.

What is claimed is:
 1. In a control system for a continuously variabletransmission for transmitting power of an internal combustion engine,the system comprising a drive pulley having a hydraulically shiftabledisc and a first hydraulic cylinder for shifting the disc, a drivenpulley having a hydraulically shiftable disc and a second hydrauliccylinder for operating the disc of the driven pulley, a belt engagedwith both pulleys, a line pressure control valve having ports and aspool and provided for supplying line pressure, a transmission ratiocontrol valve having ports and a spool, a first hydraulic circuit havinga pump for supplying oil to the first hydraulic cylinder through theline pressure control valve and the transmission ratio control valve,wherein the improvement comprises:first means for applying a firstpressure to adjacent a first end of at least one of the spools of thecontrol valves for urging said at least one spool in one direction;second means for applying a second pressure to adjacent a second end ofthe at least one spool of the control valves so as to shift said atleast one spool in the other direction; a pressure reducing valvecommunicated with the first hydraulic circuit for supplying a constantbasic pressure of the oil lower than the line pressure; a secondhydraulic circuit communicated with the pressure reducing valve forapplying the basic pressure of the oil to adjacent at least one of thefirst and second ends of said at least one spool so as to apply thebasic pressure to said at least one spool; control valve means providedin the second hydraulic circuit to control the basic pressure so as toapply the basic pressure to adjacent at least one of the first andsecond ends of said at least one spool as a control pressure; and meansfor controlling the control valve means so as to control the controlpressure in accordance with operating conditions of the engine.
 2. Thecontrol system according to claim 1 whereinthe first means is a springprovided to urge the at least one spool, and the second means isprovided for applying line pressure of the first hydraulic circuit asthe second pressure.
 3. The control system according to claim 1whereinthe control valve means is a solenoid operated on-off valvearranged to drain a part of the oil of the basic pressure.
 4. Thecontrol system according to claim 1 whereinthe first means is a springprovided to urge said at least one spool, and the second means isprovided for applying the basic pressure as the second pressure.
 5. Thecontrol system according to claim 1 wherein the pressure reducing valvecomprises a spool, a spring for urging the spool, a inlet port, a drainport and a basic pressure port.
 6. The control system according to claim5 wherein the pressure reducing valve further comprises an adjust screwfor adjusting the load of the spring.
 7. The control system according toclaim 1, whereinsaid control valve means controls the basic pressureapplied only to adjacent one of the ends of said at least one spool. 8.The control system according to claim 1, whereinsaid second hydrauliccircuit applies the basic pressure to adjacent said first and secondends of said transmission ratio control valve, and said control valvemeans controls the basic pressure applied only to adjacent one of theends of said transmission ratio control valve.
 9. The control systemaccording to claim 8, whereinsaid one of said ends is said first end.